VOLUME 102, NUMBER 35, AUGUST 27, 1998
© Copyright 1998 by the American Chemical Society
LETTERS Fabrication and Alignment of Wires in Two Dimensions S.-W. Chung, G. Markovich, and J. R. Heath* Department of Chemistry and Biochemistry, UniVersity of California at Los Angeles, 405 Hilgard AVenue, Los Angeles, California 90095-1569 ReceiVed: March 10, 1998; In Final Form: May 14, 1998
Arrays of high-aspect ratio “wires”, assembled from organically functionalized silver nanocrystals, were found to form spontaneously at the air/water interface. For all cases, the wires were 1 particle high, although the width of the wires could be controlled from 20 to 300 nm with a given set of wires characterized by a narrow (15-25%) distribution of widths. The interwire separation distance, as well as the alignment of the wires, could be controlled via compression of the wires using the Langmuir technique. The wires were found to maintain their structure and alignment up to interwire separation distances of less than 10 nm.
Techniques for directing the assembly of quantum dots (QDs) into novel superstructures have been extensively pursued over the past few years. One example is the formation of superlattices of metal and semiconductor QDs.1-6 In addition, there has also been a recent heightened interest in nanometer scale, chemically fabricated one-dimensional (1D) structures such as carbon nanotubes,7 solution-8 and gas-phase9 grown semiconductor nanowires, and metal and semiconductor wires deposited in porous templates.10-12 These 1D systems offer fundamental scientific opportunities for investigating the influence of size and shape with respect to optical,13,14 electronic,15-17 and mechanical properties.18 In addition, if such wires can be ordered and assembled into an appropriate architectural environment, then a host of nanoelectronic applications can be envisioned.19,20 Although some of the synthetic schemes for wires based on templated growth have produced ordered arrangements of nanowires in 3D, most proposed or existing electronic architectures are 2D,21 and the problem of ordering nanowires in 2D has received very little attention. In a recent paper, Zehner and co-workers utilized a phase-separated block copolymer to selectively bind Au QDs into quasi-1D arrange* To whom correspondence should be addressed: E-mail heath@ chem.ucla.edu.
ments.22 Also, Eichen and co-workers used DNA molecules to create a bridge between two electrodes and then used the DNA bridge as a template to grow a nanometer-scale, conductive silver wire.23 Presumably, such templated assemblies may provide for aligned conduction pathways in 2D. In this letter, we demonstrate a unique (nontemplated) approach to the preparation of aligned, high-aspect ratio nanometer scale wires. The approach is based on preparing a low-density Langmuir monolayer film24 of alkylthiol-passivated silver QDs. The particle monolayers are found to spontaneously assemble into lamellae or wire-like superstructures. Various lengths that characterize the system, including wire thickness, interwire separation, and the size of a domain of ordered wires, are all subject to experimental control. We prepared size-selected 3-5 nm diameter dodecanethiolcapped silver nanoparticles. The preparation and size selection of these particles is described elsewhere.24,25 Following size selection, the particles were ligand exchanged with shorter octanethiol molecules. Langmuir films were formed on a Nima Technology-type 611 Langmuir trough at 15 °C with, typically, a 1-2 mg/mL solution of particles in hexane or heptane. In most experiments we dispersed only one drop (∼3 µL, 1 mg/ mL concentration) of Ag nanoparticle solution on the water and
S1089-5647(98)01441-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/12/1998
6686 J. Phys. Chem. B, Vol. 102, No. 35, 1998
Letters
Figure 2. Low-magnification TEM micrograph of a continuous stratum structure of a compressed silver nanoparticles LB film.
TABLE 1 av particle size hexane heptane
Figure 1. TEM micrographs of typical wire-like assemblies of silver nanoparticles. The ordered wire domains extend up to micron-scale areas with varying coverage over the whole monolayer area. (A) wire structure of octanethiol-capped 34 Å (average diameter) Ag nanocrystals deposited from hexane solution. (B) Wire structure of the same particles deposited from heptane solution. (C) Wire structure of octanethiolcapped 44 Å (average diameter) Ag nanocrystals deposited from heptane solution.
transferred the nanoparticle film to a carbon-coated copper grid after the solvent of the Ag nanoparticle solution had completely evaporated. Note that only a small fraction of the water surface remained covered by macroscopic islands of monolayer film of nanoparticles after the solvent evaporates. The films were transferred as Langmuir-Schaeffer (horizontal liftoff) films. This type of transfer has been previously shown to be very effective at transferring Langmuir monolayers of quantum dots.24 The structure of the particle films was investigated using both transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM images of the assemblies of silver nanocrystals transferred from the Langmuir trough at low surface pressure (Figure 1) primarily show a wire-like structure with very narrow distribution length scales (wire width and interwire separation distance) per sample. We studied the formation of these 1D superstructures under different experimental conditions and found that the width of the wires is a function of the solvent and particle size, ranging from 20 to ca. 300 nm. The dimensions of the wire-like structures were measured for the three different sizes and two different solvents listed in Table 1. Included in the table are
width spacing width spacing
34 Å
38 Å
44 Å
80 ( 20 135 ( 35 250 ( 80 920 ( 200
40 ( 5 50 ( 20 65 ( 15 320 ( 150
25 ( 5 260 ( 45
(1) wire width and (2) interwire separation distance.26 In Figure 1, the widths of the wires formed from the heptane solution are about 3-5 times larger than those from the hexane solution, as is the interwire distance. Moreover, as the average size of the Ag nanoparticle increases, much thinner wires with a smaller interwire separation were produced. Note that in all cases there is a clear correlation between wire width and interwire separation. In Figure 2 we show the results of preparing a similar Langmuir monolayer in which we evaporated seVeral drops of the particle/hexane solution on the water surface. The monolayer was compressed to a surface pressure of 15-20 mN/m. We note several characteristics of this low-resolution TEM image. First, there is a net alignment of the wire-like arrays toward a common directionspresumably dictated by the orientation of the trough barriers. For uncompressed films, wires are typically aligned over distances of 3-5 µm. However, upon compression, a net alignment of wires over the entire TEM grid (∼1 mm) was observed. All wires that we have observed exhibited an increased alignment, accompanied by a decrease in the interwire separation distance, upon compression of the trough barriers. Most of the wires will tend to bend as they are compressed, so that the aspect ratio of the wires does not change as they align. However, some of the wires made from the largest particles will tend to fracture into 1-2 µm lengths as they align. This is consistent with the fact that the dispersion attractions between large particles are stronger than between small particles.2 In Figure 2, the wires are bent rather than fractured and the net alignment is across the diagonal of the micrograph (about a 15-20 µm field of view). In addition, we note in Figure 2 that the distribution of wire
Letters widths is quite narrow. Although the wires are narrowly spaced, adjacent wires did not coalesce together with applied surface pressure. Only at relatively high surface pressures (>20 mN/ m) did the wire structures give way to the formation of a 2D closest-packed array. The hexagonal ordering within the array depended on the size and size distribution of the particles.24 We do not understand the mechanism for the formation of these quantum-dot-based wires. It is likely that these superstructures do not represent the global free-energy minimum of the nanoparticle monolayer. This is supported by the observation that when the wires are compressed to form a closest-packed structure, the transition is not reversible. Because the interparticle potential has a spherical symmetry and the short-range dispersion attraction between the metal cores of the nanoparticles is fairly large,2 it will favor maximizing the number of nearest neighbors around each nanoparticle to form large particle islands. The persistence of the wire phase tells us, on the other hand, that long-range repulsive interactions are competing with the short-range attractions. Competing interactions have been previously shown to account for undulating phases in Langmuir monolayers of amphiphilic molecules.27,28 We have recently shown that competing interactions can be used to simulate wire formation, and we have begun trying to quantify the nature of the long-range repulsions.29 One possibility is dipolar interactions. A particle monolayer presents hydrophobic interactions to the underlying water subphase, and these hydrophobic interactions may lead to some local organization of the water molecules, which in turn can generate long-range repulsive interactions. To test this possibility, we varied the dielectric constant () of the subphase by preparing QD monolayers on both water ( ) 79) and ethylene glycol ( ) 37). We found that the subphase did not strongly affect the characteristic length scales of the assembled QD structures, implying that dipolar forces were probably not responsible for long-range repulsions. A second possibility that we have not been able to test yet is capillary forces. Our wire structures are not only easily manipulated on the water surface, but they can also be readily transferred to many different substrates for use in device applications and electrontransport measurements.30 For example, we have transferred these wires to polymer-coated Si wafers, and by treating them with chemical linkers (alkyl dithiols), we were able to selectively grow multiple (2-5) layers of Ag nanoparticles on the wires. We successfully used the thick wire patterns as lithographic shadow masks.31 In addition, we have transferred these wire structures to TEM grids, Si wafers, and graphite substrates. For the case of graphite, we have characterized the transferred structures with AFM and we find results that are consistent with the TEM images that are presented here. Our group has previously demonstrated that the optical and electronic properties of nanocrystal arrays can be controlled not only by varying the size of the organic ligands that passivate surfaces of the nanocrystals but also by compressing the monolayer to reduce the interparticle distance.32 We have recently carried out frequency-dependent electron-transport measurements on these monolayers as a function of interparticle separation distance.33 The 1D monolayer structures discussed here should exhibit appreciable electrical conductivity originating from phononactivated hopping at high temperatures and quantum-mechanical tunneling at low temperatures. This is also consistent with recent measurements from Andres’ group on supported thin films.34 One implication is that these 1D structures should provide model systems for the investigation of scaling effects in coupled quantum tunnel junctions.16,35
J. Phys. Chem. B, Vol. 102, No. 35, 1998 6687 Acknowledgment. This work was supported by the Office of Naval Research and the David and Lucile Packard Foundation. We acknowledge Prof. Bill Gelbart and Dr. Rich Sear for pointing out the significance of competing interactions in the formation of the structures discussed here. References and Notes (1) Vossmeyer, T. et al. Science 1995, 267, 1476. (2) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. ReV. Lett. 1995, 75, 3466. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (4) Harfenist, S. et al. AdV. Mater. 1997, 9, 817. (5) Pileni, M. et al. Mater. Lett. 1997, 31, 255. (6) Cusack, L.; Rizza, R.; Gorelov, A.; Fitzmaurice, D. Ang. Chem., Int. Ed. Engl. 1997, 36, 848. (7) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. Bethune, D. et al. Nature 1993, 363, 605. Tolbert, D. T. et al. Science 1994, 266, 1218. Charlier, J. C. et al. Science 1997, 275, 647. Thess, A. et al. Science 1996, 273, 483. Tans, S. J. et al. Nature 1997, 386, 474. (8) Heath, J. R.; LeGoues, F. K. Chem. Phys. Lett. 1993, 208, 263. (9) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (10) Routkevitch, D.; Haslett, T. L.; Ryan, L.; Bigioni, T.; Douketis, C.; Moskovits, M. Chem. Phys. 1996, 210, 343. Routkevitch, D.; Tager, A. A.; Haruyama, J.; Almawlawi, D.; Moskovits, M.; Xu, J. M. IEEE Trans. Electron DeVices 1996, 43, 1646. (11) Martin, C. R. Science 1994, 266, 1961. (12) Edwards, P. P.; Anderson, P. A.; Woodall, L. J.; Porch, A.; Armstrong, A. R. Mater. Sci. Eng. 1996, A217/218, 198. (13) Chang, Y.-C.; Sanders, G. D. Phys. ReV. B 1992, 45, 9202. (14) Brus, L. E. J. Phys. Chem. 1994, 98, 3575. (15) Read, A. J. et al. Phys. ReV. Lett. 1992, 69, 1232. (16) Rimberg, A. J.; Ho, T. R.; Clarke, J. Phys. ReV. Lett. 1995, 74, 4714. (17) Stafford, C. A.; Sarma, S. D. Phys. ReV. Lett. 1994, 72, 3590. (18) (a) Wong, E.; Sheehan, P.; Lieber, C. M. Science 1997, 277, 1971. (b) Treacy, M.; Ebbesen, T.; Gibson, J. Nature 1996, 381, 678. (19) Alivisatos, A. P. Science 1996, 271, 933. Yakobson, B. I.; Smalley, R. E. Am. Sci. 1997, 85, 324. (20) Heath, J. R.; Kuekes, P. J.; Snyder, G.; Wiliams, R. S. Science 1998, 280, 1717. (21) The 2D nature of most electronic architectures is at least partially related to the difficulty of addressing 3D systems. The number of wires needed to address an n-dimensional system will scale exponentially with n. (22) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 242. (23) Braun, E.; Eichen, Y.; Sivan, U.; BenYoseph, G. Nature 1998, 391, 775. (24) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B. 1997, 101, 189. (25) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (26) Wire widths and separations were measured using several TEM images, each covering a different spatial region of the sample. Interwire separation distances were measured for areas showing a short-range, parallel ordering of wires. (27) Andelman, D.; Brochard, F.; Joanny, J.-F. J. Chem. Phys. 1987, 86, 3673. (28) McConnell, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171. (29) Sear, R.; Gelbart, W. M.; Chung, S.-W.; Markovich, G.; Heath, J. R. To be published. (30) Markovich, G.; Leff, D. V.; Chung, S.-W.; Soyez, H. M.; Dunn, B.; Heath, J. R. Appl. Phys. Lett. 1997, 70, 3107. (31) Choi, S. H.; Leung, M. S.; Stupian, G. W.; Presser, N.; Chung, S. W.; Markovich, G.; Heath, J. R.; Wang, K. (to be published). (32) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (33) Markovich, G.; Collier, C. P.; Heath, J. R. Phys. ReV. Lett. 1998, 80, 3807. (34) Andres R. P. et al. Science 1996, 273, 1690. (35) Middleton, A.A.; Wingreen, N. S. Phys. ReV. Lett. 1993, 71, 3198.
